Ceramics find many applications in consumer products and in industrial or commercial products. Ceramics can be manufactured with many different characteristics. The characteristics of ceramics can vary based upon a number of factors; such as, the base composition of the ceramic, additives of the ceramic, processing conditions, and post-forming treatments.
Ceramics may be classified in many ways one of which is use. There is a group of ceramics which may be called high volume commodity ceramics. These can be considered low value added products such as brick, tile, pottery, abrasive grains, etc. At the other extreme are fine ceramics which are typically low volume speciality ceramics. Such ceramics can include (1) electronic ceramics, such as dielectrics, ferroelectrics, ferromagnetic, piezoelectrics; (2) structural ceramics which are typically fracture resistant materials such as silicon nitride (Si.sub.3 N.sub.4), silicon carbide (SiC) and toughened zirconium dioxide (ZrO.sub.2); (3) water resistant ceramics such as carbides, nitrides and borides; (4) optical ceramics and (5) bioceramics for in-body use.
The processing of ceramics is complicated by the number of steps typically required for manufacture and the need to optimize the processing steps. The processing of ceramics is extremely important because ceramics are basically flaw intolerant materials. As a result, minor chemical and physical defects can severely degrade properties. Proper selection of raw materials and processing conditions is especially important in ceramics because mistakes generally cannot be corrected during the firing and post-firing processes.
Currently many ceramics of industrial interest are made from carbides, silicon nitride and silicon oxide. While these materials are useful, there has been continuing investigation and need of more economical materials which can be processed into ceramic products without undue cost. Phosphate bonded ceramics have been suggested of interest; however, the absence of a commercially feasible process has limited their development. Thus, there has been a continuing need to provide a process which allows improved quality, while achieving more efficient production. The present invention allows the production of stronger phosphate bonded ceramic product with less flaws more economically.
In structural ceramics there has been a continual search for less expensive reagents which could be processed in a manner that would provide a commercially feasible process. The fabrication of phosphate bonded ceramics involves the reaction of phosphoric acid with a metal oxide such as zirconium. The Naval Surface Warfare Center (NSWC) developed a process in which zirconium phosphate bonded with silicon nitride (ZBPSN). This process is described in U.S. Pat. No. 5,573,986. The reaction is shown below: EQU Zro.sub.2 +2H.sub.3 PO.sub.4 (lq)+Si.sub.3 N.sub.4 .fwdarw.ZrP.sub.2 O.sub.7 +3H.sub.2 O(g)+Si.sub.3 N.sub.4
wherein lq means liquid and where g means gas. The NSWC process involved the following steps: (a) first a blend of the zirconium oxide and silicon nitride powders was prepared, (b) the blend was mixed with phosphoric acid, (c) this mixture of phosphoric acid and powder was then formed into the green body by cold isostatic pressing (CIP'ing) of the mixture, and (d) the green body was then fired to form the ceramic. The term "green body" indicates a desired shape formed from all or part of the ceramic reactants prior to the firing of that body to form the ceramic; in other words, a body formed from unfired reactants. The NSWC process has significant drawbacks. The amount of phosphoric acid required to produce a stoichiometric zirconium phosphate was very small and the acid needed to be uniformly mixed with the ceramic powders. The addition of a small amount of liquids, such as acid, to powder is difficult, and it is particularly hard to achieve mixing in such a manner that the acid is evenly distributed throughout the mixture. Mixing in accordance with the NSWC method many times produced inconsistent results and complicated the green body formation because achieving a homogeneous blend of the acid and powders was very difficult to achieve. Some of the difficulties which occurred in the NSWS phosphate bonded ceramic process were:
1. The resulting "damp" powder-acid mix had poor flow characteristics resulting in uneven packing during cold isostatic pressing. This produced density variations in the fired ceramic. This resulted in wide variations in the structural strength of the final ceramic.
2. Extensive labor was required to distribute the acid into the mixture and it was very difficult to achieve uniform distribution. As a result the fired ceramic was difficult to make homogeneous and as a result would suffer in strength and electrical performance (for those ceramics intended for electrical applications).
3. Acid being corrosive and added in the initial process steps increased the likelihood of damage to equipment, corrosion to tooling, and also possibly injury to the personnel.
The present invention has the advantages that it allows for a homogeneous mixing of the acid with the solid reactants, provides an economical process, produces a fired phosphate bonded ceramic with improved strength over prior methods, minimizes risk to personnel and equipment by handling acid, and has other advantages.